1. Field of the Invention
The present invention relates to a semiconductor technology, and, in particular, to a CMOS (Complementary Metal-Oxide Semiconductor) image sensor that has an expanded dynamic range.
2. Description of the Related Art
In recent years, high-resolution camera-equipped apparatuses, such as digital cameras, camera-equipped cellular phones, and surveillance cameras, have become widespread. As an imaging device for such a camera, a CCD (Charge Coupled Device) or a CMOS (Complementary Metal-Oxide Semiconductor) image sensor is used.
The CMOS image sensor has features of ease of manufacturing and low cost compared with the CCD, and thus it is popular in solid-state imaging. Further, a unit pixel of the CMOS image sensor is composed of MOS transistors, and thus it can be implemented in a smaller area than that of the CCD, thereby providing high resolution. In addition, signal-processing logic can be formed in an image circuit, in which pixels are formed, such that the image circuit and the signal-processing circuit can be incorporated into a single body.
Since the CMOS image sensor generally has a dynamic range of approximately 60 dB, there is a limit to generating an image in a wide illuminance range. For this reason, in a screen having a bright image and a dark image, a bright portion may be saturated and become white, and a dark portion may not be expressed.
In addition, as a digital camera or a camera-equipped cellular phone is reduced in size, low-voltage driving is performed due to demands for reducing a unit area in the pixels of the image sensor and realizing low power consumption, which makes it difficult to ensure a sufficient dynamic range.
In the related art, in order to solve the above-described problems, the structure shown in FIG. 1 is used to expand the dynamic range of the image sensor.
FIG. 1 is a circuit diagram showing a unit pixel having a general 4-T structure in a CMOS image sensor.
Referring to FIG. 1, the pixel having a 4-T structure is composed of one photodiode (PD) 110, and four NMOS transistors, that is, a transfer transistor (Tx) 120, a reset transistor (Rx) 122, a drive transistor (Dx) 124, and a select transistor (Sx) 126.
In a state where the transfer transistor (Tx) 120 is turned off, if light is irradiated onto the surface of the photodiode (PD) 110, holes and electrons are separated. Then, the holes flow to a ground to be then removed, and electrons accumulate in the photodiode (PD) 110.
The transfer transistor (Tx) 120 functions as a transmission channel to apply a predetermined voltage to a gate 121 of the transfer transistor (Tx) 120, and to transfer the electrons accumulated in the photodiode (PD) 110 by light to a floating diffusion region (FD) 130. Further, the transfer transistor (Tx) 120 performs a reset function to completely remove the electrons from the photodiode (PD) 110.
The reset transistor (Rx) 122 resets the floating diffusion region (FD) 130 by setting the potential of the floating diffusion region (FD) 130 to a desired value and eliminating charge. That is, the reset transistor (Rx) 122 eliminates the charge that has accumulated in the floating diffusion region (FD) 130 for signal detection.
The drive transistor (Dx) 124 operates according to the charge accumulated in the floating diffusion region (FD) 130, and functions as a buffer amplifier having the configuration of a source follower. The select transistor (Sx) 126 is switched for addressing.
If charge accumulates in the photodiode (PD) 110, a high voltage is applied to a gate of the reset transistor (Rx) 122 to set the voltage of the floating diffusion region (FD) 130 to VDD, and then a corresponding voltage value is read. Next, a high voltage is applied to the gate of the transfer transistor (Tx) 120 to transfer the charge that has accumulated in the photodiode (PD) 110 to the floating diffusion region (FD) 130, a corresponding voltage value is read, and subsequently a difference between the read voltage values is read.
In this structure, in order to expand the dynamic range, the capacitance of the floating diffusion region (FD) 130 is increased to receive the charge from the photodiode (PD) 110 without overflow.
However, if the capacitance is increased, sensitivity of the CMOS image sensor is decreased, and a dark image may not be expressed. Therefore, it is not desirable to simply increase the capacitance of the floating diffusion region (FD) 130.